Friction brakes are widely used on land and air vehicles as well as with stationary machinery. The following discussion will be largely in the context of the demanding use with high performance land vehicles, but the contemplated utility is with all friction brakes.
Broadly defined, friction brakes function by pressing a friction pad against a moving surface thus converting kinetic energy into thermal energy. Many embodiments of such brakes are well known in the art. The moving surface can be a disc, drum, wheel surface etc., but will be herein referred to generically as a rotor. As is known, the movable friction pads may be carried by calipers or on expanding shoes or the like. Known pad actuating means include hydraulic, pneumatic, mechanical, electromagnetic actuators, etc. In all instances the operation of these braking mechanisms will generate thermal energy, often including large quantities of thermal energy, while slowing or stopping the vehicle, thus reducing or diminishing kinetic energy. The ability to dissipate such thermal energy to the atmosphere is typically the ultimate limiting factor in the effectiveness of a given braking assembly or mechanism, with the heat sink capacity of the assembly being the peak-use-limiting factor. The following discussion will be primarily in the context of the ubiquitous disc/caliper brake structure. However, those skilled in the art will readily recognize that the principles are equally applicable to other iterations of friction brakes.
In a given braking occasion, the rotor ultimately dissipates most of the thermal energy to the atmosphere, serving initially as a heat sink and ultimately as a member to transfer the thermal energy to the atmosphere through radiation and convection. Friction pads are of lesser mass and in thermal contact with various structures adversely affected by heat. Accordingly, the friction pad is of low thermal conductivity in order that the thermal energy preferentially flows into the rotor. In all brake assembly components and friction pads in particular, wear is accelerated by heat inherently generated by frictional contact. This heat is carried as the hot gases and particles present within the rotor boundary layer are passed on to or delivered to the leading edge of the friction pad.
Rotor cooling conventionally relies primarily upon convection utilizing the relatively large area of the rotor body. Both vehicle motion and rotation of the rotor moves this body through the air. Often internal rotor fins, which both draw cool air to the rotor and which move through the air to generate turbulence and enhance convection have been used to augment convective cooling. However, to deal with the extreme temperatures developed at the friction pad, the rotor first serves as a heat sink to distribute the thermal energy throughout the mass or structure of the rotor body. For better management of the heat, rotor bodies tend to have a large mass.
Cool air from a vehicle slipstream, or from a blower, can be ducted to the rotor to improve cooling. Often, when air is to be ducted to the rotor, the brake requirements are typically demanding, as in a racing or high performance vehicle. In such instances the rotor is also large to serve as a heat sink and internally finned to facilitate thermal energy transfer to the atmosphere.
By way of example, a typical automotive disc brake includes a cylindrical rotor disc of eight to twelve inches in diameter, the disc being acted upon on opposite sides by opposed friction pads carried in calipers and activated by a hydraulic circuit. Certain rotor discs may be solid, with these being used in relatively light duty applications since solid rotors tend to be of lower thermal mass and modest surface area, and thus are more easily warped by high temperatures. More typically, the disc rotor will be ventilated, i.e. have an open center bridged by fins to pump air through the disc rotor and reject or dissipate a major portion of the brake heat through such fins. Such disc rotors essentially conduct heat from the surfaces which contact the friction pads, to the finned interior of the disc rotor structure. Accordingly, the disc must be heavy to serve as a heat sink and to provide structural strength in areas between the friction pads and the bridging fins.
Weight is generally undesirable in any vehicle, with rotating, unsprung weight being particularly objectionable. Brakes usually constitute unsprung weight which compromises fuel efficiency, ride and handling. The energy expended to spin a heavy rotor is generally wasted and adds to the overall energy that the brakes must convert to thermal energy.
In a worst case instance, such as an aircraft during landing, the craft or vehicle may be brought to a sudden stop from a high velocity. Without the benefit of a slipstream and moving rotor fins when stopped, only the thermal mass of the rotors and adjacent structure are available to serve as a heat sink to absorb the massive quantity of thermal energy generated by such an event. Accordingly, such rotors must have considerable mass, and thus be heavy.
A land vehicle descending a long grade must deal with potential energy that, in time, can easily create an amount of thermal energy sufficient to swamp the heat sink capabilities of a rotor. Heat rejection through radiation and convection are the only means for preserving the effectiveness of the brakes. Braking effectiveness is easily compromised and may be lost under adverse heat conditions.
In particular, under certain conditions a boundary layer of fluid moving with the rotor (i.e., a layer of fluid that actually adheres to the rotor braking surface during rotor rotation) may become heated to extreme temperatures along the swept area and carries a substantial quantity of thermal energy during aggressive braking. This ring of extremely hot air is known as the “ring of fire”. The boundary layer, or ring, may also carry significant amounts of friction pad particulate. In conventional braking assemblies, this boundary layer travels with the rotor until it is disrupted by the leading edge of the friction pad. While inherent in such conventional designs, this is highly undesirable for several reasons. The rotor has a much higher coefficient of thermal conductivity than that of the layer of superheated air. Thus thermal energy in the superheated “ring of fire” is to a substantial degree transferred to the rotor, but, with rotor travel, is also returned to the leading edge of the friction pad to heat the pad and its actuating structure. Episodes of brake fade are increased and pad wear is substantially accelerated at high temperatures. Hydraulic fluid lodged in lines located near actuating mechanisms is subject to boiling.
The boundary layer phenomenon is well known. At the interface between a fluid and the surface of a solid, molecular attraction bonds the initial layer or film of fluid firmly to this surface. Subsequent layers of fluid molecules are cohered to the initial layer. Thus a relatively thin but appreciable volume of fluid is formed. This adhered boundary layer volume moves with the solid.